With the advent of modern antipollution laws in the United States and around the world, significant and new methods of minimizing various pollutants are being investigated. The burning of fuel--be the fuel wood, coal, oil, or a natural gas--likely causes a majority of the pollution problems in existence today. Certain pollutants, such as SO.sub.2, are created as the result of the presence of a contaminant in the fuel source and may be removed either by treating the fuel to remove the contaminant or by treating the exhaust gas eventually produced. Other pollutants (such as carbon monoxide) are created as the result of imperfect combustion and may be removed by post-combustion oxidation or by improving the combustion process. The other principal pollutant, NO.sub.x (an equilibrium mixture mostly of NO but also containing very minor amounts of NO.sub.2), may be dealt with either by controlling the combustion process to minimize NO.sub.x production or by later removal. Removal of NO.sub.x, once produced, is a difficult task because of its relative stability and its low concentration in most exhaust gases. One solution found in automobiles is the use of carbon monoxide chemically to reduce NO.sub.x to nitrogen while oxidizing the carbon monoxide to carbon dioxide. However, in some combustion processes (such as in gas turbines) the carbon monoxide concentration is insufficient to react with and to remove the NO.sub.x.
It must be observed that unlike the situation with sulfur pollutants where the sulfur contaminant may be removed from the fuel, removal of nitrogen from the air fed to the combustion process is clearly impractical. Unlike the situation with carbon monoxide, improvement of the combustion reaction would likely increase the level of NO.sub.x produced due to the higher temperatures present in the combustion process.
Nevertheless, the challenge to reduce NO.sub.x remains and several different methods have been suggested. The NO.sub.x abatement process chosen must not substantially conflict with the goal for which the combustion gas was created, i.e., the recovery of its heat value in a turbine, boiler, or furnace.
Many recognize that a fruitful way of controlling NO.sub.x production is to limit the localized and bulk temperatures in the combustion zone to something less than 1800.degree. C. See, for instance, U.S. Pat. No. 4,731,989 to Furuya et al. at column 1, lines 52-59 and U.S. Pat. No. 4,088,135 to Hindin et al. at column 12.
There are a number of ways of controlling the temperature, such as by dilution with excess air, controlled oxidation using one or more catalysts, or staged combustion using variously lean or rich fuel mixtures. Combinations of these methods are also known. One widely attempted method is the use of multi-stage catalytic combustion. Most of these processes utilize multi-section catalysts of metal oxide on ceramic catalyst carriers. Typical of such disclosures are:
TABLE __________________________________________________________________________ Country Document 1st Stage 2nd Stage 3rd __________________________________________________________________________ Stage Japan Kokai 60-205129 Pt-group/Al.sub.2 O.sub.3 & SiO.sub.2 La/SiO.sub.2.Al.sub.2 O.sub.3 Japan Kokai 60-147243 La & Pd & Pt/Al.sub.2 O.sub.3 ferrite/Al.sub.2 O.sub.3 Japan Kokai 60-66022 Pd & Pt/ZrO.sub.2 Ni/ZrO.sub.2 Japan Kokai 60-60424 Pd/- CaO & Al.sub.2 O.sub.3 & NiO & w/noble metal Japan Kokai 60-51545 Pd/* Pt/* LaCoO.sub.3 /* Japan Kokai 60-51543 Pd/* Pt/* Japan Kokai 60-51544 Pd/* Pt/* base metal oxide/* Japan Kokai 60-54736 Pd/* Pt or Pt--Rh or Ni base metal oxide or LaCO.sub.3 /* Japan Kokai 60-202235 MoO.sub.4 /- CoO.sub.3 & ZrO.sub.2 & noble metal Japan Kokai 60-200021 Pd & Al.sub.2 O.sub.3 /+* Pd & Al.sub.2 O.sub.3 /** Pt/** Japan Kokai 60-147243 noble metal/heat resistant carrier ferrite/heat resistant carrier Japan Kokai 60-60424 La or Nd/Al.sub.2 O.sub.3 0.5% SiO.sub.2 Pd or Pt/NiO & Al.sub.2 O.sub.3 & CaO 0.5% SiO Japan Kokai 60-14938 Pd/? Pt/? Japan Kokai 60-14939 Pd & Pt/refractory ? ? Japan Kokai 61-252409 Pd & Pt/*** Pd & Ni/*** Pd & Pt/*** Japan Kokai 62-080419 Pd & Pt Pd,Pt & NiO Pt ot Pt & Pd Japan Kokai 62-080420 Pd & Pt & NiO Pt Pt & Pd Japan Kokai 63-080848 Pt & Pd Pd & Pt & NiO Pt or Pt & Pd Japan Kokai 63-080849 Pd, Pt, NiO/? Pd & Pt (or NiO)/? Pt or Pd & __________________________________________________________________________ Pt/? *alumina or zirconia on mullite or cordierite **Ce in first layer; one or more of Zr, Sr, Ba in second layer; at least one of La and Nd in third layer. ***monolithic support stabilized with lanthanide or alkaline earth metal oxide Note: the catalysts in this Table are characterized as "a"/"b" where "a" is the active metal and "b" is the carrier
It is, however, difficult to control intermediate or between-stage temperatures in these processes. Since the object of each of the processes is to produce a maximum amount of heat in a form which can be efficiently used in some later process, the combustive steps are essentially adiabatic. Consequently, a minor change in any of fuel rate, air rate, or operating processes in an early stage will cause significant changes in the inter-stage temperatures. Very high temperatures place thermal strain on downstream catalytic elements.
This list also makes clear that platinum group metals (including palladium) are considered useful in catalytic combustion processes. However, conventional catalytic combustion processes often mix the fuel and air and then pass this mixture over a catalyst with essentially complete combustion in the catalyst bed. This results in extremely high temperatures, typically 1100.degree. C. to 1500.degree. C. For this reason, much of the catalyst development work is directed at catalysts and supports that can withstand those high temperatures and yet remain active. Some have relied on process control schemes in which the flow rate of an intermediate stream of air or fuel is introduced between catalyst stages and is controlled based upon bulk gas temperature. Furuya et al. (mentioned above) describes one approach in circumventing the problems associated with a high catalyst temperature through dilution of the fuel/air mixture with air fed to the catalyst so that the resulting mixture has an adiabatic combustion temperature of 900.degree. C. to 1000.degree. C. This mixture is passed through the catalyst and partial or complete reaction gives a maximum catalyst temperature less than 1000.degree. C. and a gas temperature less than 1000.degree. C. Additional fuel is added after the catalyst and homogeneous combustion of this mixture gives the required temperature (1200.degree. C. to 1500.degree. C.). This process, however, suffers from the need to add fuel at two stages and the requirements to mix this additional fuel with hot gases without obtaining a conventional high temperature diffusion flame and the associated production of NO.sub.x.
The process of this invention mixes air and fuel at the beginning of the combustor in a ratio such that the final combustion temperature is, after further combustion step(s), that required by some later process or device which recovers the heat from the combustive gas, e.g., a gas turbine. A typical mixture might be methane and air at a volume fuel/volume air ratio of 0.043. Such a mixture (after being preheated to 350.degree. C.) would provide a combustion temperature of about 1300.degree. C. This mixture passes over a catalyst and is only partially combusted with the catalyst limiting the maximum catalyst temperature to less than about the thermogravimetric analysis (TGA) transition temperature of the reaction: EQU Pta.fwdarw.Pd+1/2O.sub.2
at the partial pressure of oxygen present during the reaction. As a rule of thumb, this TGA transition temperature is approximately 780.degree. C. to 800.degree. C. in air at one atm and 930.degree. C. to 950.degree. C. in air at ten atm.
The temperature self-controlling feature of this invention takes place by employing one or more of the following:
a. Use of palladium as the active catalytic metal since the active palladium oxide species interconverts to less active palladium metal as the temperature rises to the palladium oxide/palladium TGA transition temperature. PA1 b. Use of a diffusion barrier applied over the catalyst surface to limit the rate at which the fuel diffuses to the catalyst and, therefore, limits the catalytic reaction rate and allows palladium to limit the maximum temperature.
The interconversion of palladium oxide and palladium at approximately 800.degree. C. has been described previously (for example, by Furuya et al. in U.S. Pat. No. 4,731,989). However, this patent describes this interconversion as a disadvantage since the active palladium oxide species is converted to a less active palladium species thus preventing the combustion reaction from going to completion on the catalyst. The inventive process of this patent uses this palladium oxide/palladium interconversion to limit the catalyst temperature and thereby permit the use of high activity yet thermally stable catalysts.
By maintaining the catalyst temperature at or below the palladium oxide/palladium TGA transition temperature, problems associated with thermal sintering of the catalyst, vaporization of the palladium, and thermal shock of the support can be minimized or eliminated.
We strongly prefer to use a monolithic metal support for the palladium-containing catalyst.
The use of metal catalyst supports for platinum group metals has been suggested in passing. See, for instance, U.S. Pat. No. 4,088,435 to Hindin et al., "platinum group metals" at column 4, lines 63 et seq., and "the support may be metallic or ceramic . . . " at column 6, line 45. Conversely, the use of a platinum group alloy monolithic catalyst as a combustion catalyst is suggested in U.S. Pat. No. 4,287,856 to Enga at column 1, line 65 et al. Other similar disclosures are found in the earlier U.S. Pat. Nos. 3,966,391; 3,956,188; 4,008,037; and 4,021,185 all to Hindin et al. Platinum on a steel ("Fecralloy") support as a combustion catalyst for low heating value gas is suggested in U.S. Pat. No. 4,366,668 to Madgavkar et al.
Other disclosures of metals and metal supports used mainly for automotive catalytic converters include:
______________________________________ Country Document Patentee ______________________________________ U.S. 3,920,583 Pugh U.S. 3,969,082 Cairns et al. U.S. 4,279,782 Chapman et al. U.S. 4,318,828 Chapman U.S. 4,331,631 Chapman et al. U.S. 4,414,023 Aggen et al. U.S. 4,521,532 Cho U.S. 4,601,999 Retallick et al. U.S. 4,673,663 Magnier U.S. 4,742,038 Matsumoto U.S. 4,752,599 Nakamura et al. U.S. 4,784,984 Yamanaka et al. Great Britain 1,528,455 Cairns et al. ______________________________________
As a group, these patents generally discuss ferritic catalyst supports upon which alumina is found as micro-crystals, coatings, whiskers, etc. Many disclose that platinum group metals are suitably placed on those supports as catalysts. None suggest the ability of palladium to limit the catalyst temperature or the use of a diffusion barrier on the exterior of the catalyst as an aid to the catalyst operation.
Moreover, in a practical sense, the use of metal substrates has been limited to applications where the adiabatic combustion temperature is below 1100.degree. C. or 1000.degree. C. and where the complete combustion of the fuel/air mixture will result in a substrate temperature that would not damage the metal. This limitation caps the final gas temperature that can be achieved or requires the use of staged fuel or air addition further complicating the combustor design. The use of the inventive process limits the metal substrate temperature to less than 850.degree. C. at one atm pressure and to less than 950.degree. C. at 16 atm pressure even for fuel/air mixtures with adiabatic combustion temperatures up to 1500.degree. C.
By limiting the substrate temperature, the inventive process also offers advantage for ceramic substrates since limiting the substrate temperature reduces thermal stress and the propensity for structural failure due to thermal shock during start-up and shutdown of the combustor. This protection is especially important for fuel/air ratios corresponding to adiabatic combustion temperatures of 1300.degree. C. to 1600.degree. C.
In summary, although the literature suggests various unrelated portions of the inventive process and the catalyst structure, none of these documents suggests that a palladium catalyst (either with or without an external diffusion layer) can offer advantage by limiting the substrate temperature nor is such a structure suggested.